In an electrophotographic process, an electric latent image is formed on an image holding member, and the latent image is developed with a toner to form a toner image, which is transferred to a transfer material, such as paper, and then fixed by such measures as heat and pressure. The toner used herein includes not only a conventional monochrome black toner, but also toners of plural colors for forming a full color image.
The toner is used as a two-component developing agent after mixing with carrier particles or as a one-component developing agent, which may be a magnetic toner or a non-magnetic toner. The toner is generally produced by a kneading and pulverization method. In the kneading and pulverization method, raw materials, such as a binder resin, a pigment, a releasing agent, such as wax and a charge controlling agent, are melt-kneaded to form a mixture, and the mixture is cooled, then finely pulverized and classified to produce target toner particles. To the surface of the toner particles produced by the kneading and pulverization method, inorganic fine particles and/or organic fine particles are added to provide a toner.
The toner particles produced by the kneading and pulverization method have an irregular shape and heterogeneous surface composition. The shape and the surface composition of the toner particles are slightly changed by the pulverizing property of the materials used and the conditions upon pulverizing, but are difficult to control purposefully.
The use of a material that has higher pulverizing property provides the following problems. The toner particles are further pulverized to fine particles due to various kinds of stress in a developing device to change the shape thereof. As a result, in a two-component developing agent using the toner particles, the finely pulverized toner particles are firmly adhered to the surface of the carrier to accelerate deterioration in charging property of the developing agent. In a one-component developing agent using the toner particles, the particle size distribution is broadened, whereby the finely pulverized toner particles are scattered, and the developing property is deteriorated due to change in shape of the toner, which bring about deterioration in image quality.
A toner containing a releasing agent, such as wax, may suffer pulverization at the boundary between the binder resin and the releasing agent, which may cause exposure of the releasing agent on the surface of the toner. In particular, a toner containing a resin that has high elasticity and is hard to be pulverized and brittle wax, such as polyethylene wax, frequently suffers exposure of the polyethylene wax on the surface of the toner. The toner is advantageous in releasing property upon fixing and in cleaning property of the non-transferred toner from a photoreceptor, but the polyethylene wax on the surface of the toner may be released from the toner by a mechanical force, such as a shearing force, in a developing device and may be easily transferred to a developing roller, an image holding member, a carrier and the like. Accordingly, the developing roller, the image holding member, the carrier and the like are easily contaminated with the polyethylene wax, which may cause deterioration in reliability of the developing agent.
Under the circumstances, as a production method of a toner having a shape and a surface composition that are purposefully controlled, an emulsion polymerization and aggregation method is proposed in JP-A-60-225170, JP-A-63-282749 and JP-A-6-282099.
In the emulsion polymerization and aggregation method, a resin dispersion liquid is once produced by emulsion polymerization, and a colorant dispersion liquid containing a colorant dispersed in a solvent is separately produced. The dispersion liquids are mixed to form aggregated particles having a diameter corresponding to the particle diameter of the target toner, and the aggregated particles are fused by heating to provide toner particles. According to the emulsion polymerization and aggregation method, the shape of the toner particles can be arbitrarily controlled from an irregular shape to a spherical shape by selecting a heat temperature condition.
In the emulsion polymerization and aggregation method, toner particles can be obtained by aggregating and fusing at least a dispersion liquid of resin fine particles and a dispersion liquid of a colorant in a predetermined condition. However, the emulsion polymerization and aggregation method involves limitation in kind of a resin that can be used, i.e., a styrene-acrylic copolymer is suitable for the method, but a polyester resin, which is known as a resin excellent in fixing property, cannot be applied to the method.
As a production method of a toner using a polyester resin, a phase inversion emulsification method is known, in which a colorant dispersion liquid and the like are added a solution of a binder resin dissolved in an organic solvent, and water is added to the mixture, but in this method, it is necessary to remove and recover the organic solvent. JP-A-9-311502 proposes a method that can produce fine particles in an aqueous medium by mechanical agitation without the use of an organic solvent, but in this method, it is necessary to feed a resin or the like in a molten state to an agitating device, which brings about difficulty in handling. Furthermore, this method is low in degree of freedom upon controlling the shape, and thus the shape of the toner cannot be arbitrarily controlled from an irregular shape to a spherical shape.
JP-A-2007-323071 proposes a method that solves the aforementioned problems, in which materials constituting a toner are melt-kneaded or mixed, and the mixture in a molten state under heating is mechanically formed into fine particles, which are then aggregated to produce a toner. This method can provide a developing agent that has a small particle diameter, a controlled shape, less fluctuation in surface composition, and favorable fixing property and image quality, without the use of an organic solvent.
However, when a batch type high-speed agitation device is used as a device for forming fine particles in this method, the polyester resin is formed into fine particles under a high temperature and high alkaline state, whereby the polyester resin may be hydrolyzed to deteriorate the fixing property while depending on the kind of the polyester resin.
The use of a high-pressure device for forming fine particles is advantageous since the polyester resin can be processed continuously and can be suppressed from being hydrolyzed by shortening the period of time when the polyester resin is under a high temperature and high alkaline state. However, the particles may be grown to coarse particles upon cooling and may stay in a pipe of the high-pressure device for forming fine particles.